Automatic telephone-exchange system



B. G DUNHAM AUTOIATIC TELEPHONE EXCHANGE SYSTBI [glad Oct. 5, 1904 6 Sheets-Sheet 1 a. G. DUNHAM AUTOIATIC TELEPHONE EXCHANGE SYSTSI Filed Oct'. 5, 1904 6 Sheets-Shoat 4' Dec. 29.1925- B. G. DUNHAM AUTOIA'IIC TELEPHONE EXCHANGE SYSTBI Filed Oct. 5, 1904 6 Sheets-Shoat 5 WITAESSES:

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jip e v Patented Dec. 2%, W925.

Mar Price.

BERT Gr. DUNHA'M, OF CHICAGO, ILLINOIS, ASSIGNOR, BIZ MESNE ASSIGNMENTS, TO WESTERN ELECTRIC COMPANY, INCOBIPOE-ATED, A CORPORATION OF NEW' YORK.

AUTOMATIC TELEPHOI TE-EXCHANGE SYSTEM.

Application filed October 5, 1964.

To all whom it may concern:

Be it known that I, BERT G. DUNHAM, a citizen of the United States of America, and a resident of Chicago, county of Cook, and State of Illinois, have invented a new and useful Improvementin Automatic'Telephone' Exchange Systems, of which the following is a specification.

My invention relates to automatic telephone exchange systems; that is, those systems wherein the connection between any two lines is made by automatic switches at the central oi'hce, these switches being directed in their action by mechanism at the subscribers station under the control of the subscriber who desires to make a call.

In systems of the kind generally installed at the time of my invention, all the vertical and rotary relays of all the switches involved in a connection, are included in series in their respective line wires, and as a result the ta king circuit built up as I have described, contains these relays in series. In a system adapted to serve all numbers under ten thousand, there would be a total of eight electromagnets in series in each talking circuit between two subscribers. This is dis advantageous because of the deleterious etrect on conversation caused by the impedance of the magnets and also because of certain limitations as to the strength and quickness of the magnets due to the method of winding necessarily imposed for securing tolerable conversation. In another invention for which I am making application for a patent have eliminated the series magnets and have arranged all of the controlling magnets in bridges across the line. In other applications, I have disclosed systems where in the number of bridges across the line is somewhat reduced from that required in a co-pending application and it is one of the main purposes of the system of this application to still further reduce the number of bridges present across the combined circuit ct two connected lines, and at the same time to atlford a further improvement in the method by which those switches involved in a connection which have no bridged magnets present, might be released after conversation by tiie action of one of the switches at which a bridged magnet is present, thus securing a more perfect talking circuit and a more positive and reliable action of the relays.

Serial No. 227,289.

My invention is illustrated in the accompanying sheets of drawings, which form a portion of this application, and in which- Fig. 1, is a simplified circuit showing the connection of the relays with respect to the talking circuit, when a complete connection has been established;

Fig. 2, is a representation of the circuits at the subscribers station adaptable for use with my invention;

Fig. 3, shows the circuits of the calling subscribers selector switch;

Fig. 4-, the circuits of a second selector switch;

Fig. 5, those of a connector switch;

Fig. 6, the circuits of a first selector switch of a line that is being called;

Figure 7 is a diagrammatic representation of the features of the side-switch employed.

Like characters refer to throughout the several views.

An understanding of the circuits of the subscribers station may be had by reference to Fig. 2. The talking apparatus, consisting oi a receiver, transmitter and induction coil, may be connected between the vertical and rotary limbs of the lines, V and R. respectively, by the action of the hook switch, 3, in a manner well understood. Under normal conditions, that is, when the receiver is upon the hook, the bell or ringer, i, is connected between the two sides of the line, this connection extending through the spring, 5, and the arm, 6, 01" the hook when the telephone is not in use. When the receiver is removed from the hook this connection is broken, and the talking contacts completed in an obvious manner. All ground connections at the subscribers station are primarily controlled at the pair of contacts, 7, these contacts being held open as long as the receiver is on its hook, and after the receiver has been removed, until the impulse transmitting device has been put into operation.

The impulse transmitting device acts by causing the springs, 8 or 9, to engage the ground spring, 10, according to whether impuls s are to be sent over the vertical or rotary sides of the line. That is, it sir: impulses are to be transmitted, the spring, 8, will be pressed against the spring, 10, six times, thus allowing that many impulses to flow from the central oiiice over the vertical like parts side of the line to ground. It is obvious from the presence of the spring, 7, that no such impulse can take place unless the re ceiver has been removed from its hook, and the impulse transmitter operated, for only under these conditions can the contact between springs, 7, be closed. In a similar manner an impulse over the rot ry side of the line may be secured by pressing the spring, 9, against the spring, 10. It should be remembered that all selecting is done over the vertical side of the line by sending aseries of impulses corresponding to the digits belonging to the number of the subscriber wanted. Each series of impulses over the vertical side of the line caused by the movements of the spring, 8, is followed by a single impulse over the rotary side of the line cause-d by the movement of the spring, 9. lVhen the hook, 3, is depressed at the close of the conversation, its arm, 11, serves to press the springs, 12, 13 and 1s, momentarily into engagement with each oth r, thereby grounding both the vertical and ro tary sides of the line through the spring, 14, and the contact, 7. The mechanical arrangement of parts is such that the springs, 12, 13 and 14 are not brought into engagement with each other as the hook rises, or while it is either depressed or elevated.

15 is the spring actuated by the ringing button, this spring normally bearing against spring, 16, and serving to hold closed the talking circuit. When, however, the ringing button is depressed, spring, 15, engages spring, 17, thereby grounding the vertical side of the line for the operation of the ringing relay in the connector switch.

Referring now to Fig. 8, the limbs, V and R, extending from the subscribers stat-ion shown in Fig. 2, may be seen entering the left of Fig. 3, where they are similarly lettered.

B is a source of direct current, such as a battery or suitable generator. VR and RR are relays normally associated with the two sides, V and R, of the line. Upon the opera tion of the subscribers switch, with reference to the first digit of the number he is calling, a series of ground contacts will be made with the vertical wire. Current from the battery, B, will pass through the conacts, 19, and 21, of the relay, CU, to the relay, VB, thence to the line by contact of the lever, with its point, 26". The parts, 25 and 26", are of a device known as a side switch, whi h is composed of the levers, 25 to 25 inclusive, and their contacts, 26 to 26, 27 to 27 and 28 to 28, inclusive,

Figure 7 illustrates diagrammatically the mechanical features of a ide-switch adapted o carry out the operations attributed to it. In this figure the same reference char actors :are used, Where possible, as those used in the other figures. No attempt is made,

in this figure, however, to illustrate associated parts which are not directly concerned in the operation of the side-switch. Thus, for example, the private magnet, PM, is shown without the contacts 29, 30 and which, in fact, are associated with this niagnetas shown, for example, in Fig. 3. The rotatably mounted shaft, 201, carries the side switch levers 25 25 etc., only two of which are shown, however. The assoriated contacts are shown at 20 27 28, ml at 26 27*, and 28 The arm, or lever, 02, having a triangular cross-section is rigidly mounted upon the shaft, 201. The private magnet PM is provided with an armature, 203, pivoted at 20-1 and having an arm 205, extending toward the lever, 202. spring, 211, co-operating with a spring arm, 212, tends to cause a rotation of the side-switch shaft and its switch arms in the direction of the arrow, 213. Thus upon the energization of the private magnet PM, he an 205, is depressed and this causes the tooth, 206, on the escapement arm to strike Lee top of the tooth, 210, on the spring in niber, 208, this member because. of its r ngy characteristics being held by its engement with lever 202 and not, therefore, ng effected by the movementof the armane l v' hen the arm 205 is depressed LO cause the tooth 206 thereon to strike the top of the tooth 210 on the spring member 208, the spring member is thus effectively forced out of engagement with the escapement arm, 202. The face of the tooth, 206, is only slightly to the left of the face of the tooth, 210, so that the substitution of the tooth, 206,to retain the escapement arm $02 in its position in place of the teeth 210 permit any material rotation of the e-switch shaft due to the tension of the mg, 211. lVhen, however, the de-eneration of the private magnet, PM, per- 1 retraction of the armature, 203, due to the influence of the spring, 214, the face of the tooth, 210, will not engage the front ace of the escapement arm, 202, and arm and shaft will be permitted to turn to the left under the influence of the spring, 211, until it strikes the face of the tooth, 200. So, also, a subsequent energization of ri'vate magnet, PM, will first cause the to be substituted for the tooth, to retain the side-switch shaft in its demand then upon the de-energization magnet the side-switch shaft l to move a step further in adie wire, 215, which is connected h the arm, 212, passes through a hole in the rigidl supported guide, 216, and is provided with an adjustable stop, 217, by means of wh ch the forward rotation of the e-switch shaft is limited.

The restoration of the side switches and the shaft; upon which they are mounted to does not iii their normal position is brought about by the energization and subsequent de-energiza- 31, to its normal position, the armature, 218,.

of the magnet, DM, is provided with a catch, 219. The armature is pivoted at 220 and returned to its normal position by a comparatively stifi spring, 221. The wire, 215, which is quite springy, is provided at its end with a very thin, flexible plate, 222, this plate having a suitable opening at 223. The vertical magnet, VM, is equipped with an armature, 224, pivoted at 225, and pro vided at its extremity with a catch, 226. lhe energization of this magnet, VM, raises the armature, 224i, and by means of the catch, 226, itraises also the plate, 222, disengaging it from the. restraining influence of the catch, 219, and making the rotation of the side-switch shaft, 201, subject only to the operation of the escapement associated with the private magnet, PM.

T he restoration of the selector mechanism upon the energization and de-energization ot the disconnect-magnet, DM, restores the plate, 222, to the position shown in Fig. 7. When next this particular mechanism is called into service. the first energization of vertical magnet, VM, immediately releases the plate, 222, from the catch, 219, and puts the side-switch under the control or" the escapement associated with the private magnet, PM.

The relay, VR, will respond in impulses of its armature corresponding in number with the ground contacts at the substation with the wire, V. It will result that current from the battery, B, will flow through the vertical magnet, VM, the contact of springs, 29 and 30 of the private magnet, PM, and springs, 30 and 31 of the relay, VB, to ground, actuating the vertical magnet, VM, in as many impulses as there are digits in the called figure. The shaft, 32, will be lifted by the pawl associated with the armature of the magnet, VM, and will stop opposite an arc of contacts in the contact bank, which are contains trunks leading to the selectors of the thousand indicated by the digit that is called by the subscriber. As each series of vertical impulses is followed by one rotary impulse, the single ground contact at the substation upon the wire, R, will cause current from the battery, B, through the relay, RR, to actuate the latter one impulse. This will energize the magnet, PM, drawing up its armature, and releasing it, making contact between the springs of that magnet. These contacts are immaterial at this time. The important thing which happens is that by the double. action of drawing up and talling away, the armature of the magnet, PM,

acts, through the escapcinent connecting it to the side switch, to advance the levers, 25 to 25' inclusive, one step. One of the re-ults of making this step is the establishment of a circuit from the battery, B, through the rotary magnet, RM, to ground, through the lever, 25, and the point, 27. As the circuit of the rotary magnet, RM, contains the contact, 34, which Contact is l')1'()l611 when the armature of the magnet is drawn up after the manner of the familiar vibrating bell, the armature ot' the magnet, RM, will execute a series of strokes, acting through a pawl, to rotate the shaft, 32. This will more the wipers, and 36. in the arc of the contact bank before which they have been brought by the vertical motion and will. more the contact wiper, 37, in a similar 2111". The wiper, 37, is adapted to engage contacts of the third wire associated with the talking conductors of the trunk, and it is intended that upon an engagement between a contact of such third wire and the wipers, 37, further rotation of the shaft shall be permitted, if such contact is grounded; but shall be prevented if it is not grounded, as it is the presence of the ground that indicates that the trunk is busy. Ii successive contacts so engaged by the wiper, 3?, in the rotation of the shaft prove to be grounded, current from the battery, B, through the private magnet, PM, the contact, 27, the lever 25, the disconnect relay, DR, and the wiper, 37, will energize the magnet, PM, which is of relatively high resistance, but will not energize the relay, DR, because it is of few turns, and low resistance. As successive grounded contacts are engaged by the wiper, 37, the armature of the magnet, PM, will continue to be held. As soon as a contact which is not grounded is taken by the wiper, 37, the armature of the magnet, PM, will be released, and by falling back will act, through its escapement, to advance the. side switch levers to their third and extreme position. The breaking of contact between the point, 27 and 25, will reopen a circuit of the magnet, RM, stopping further rotative effort. The movement of the levers, 25 and 25, upon their contacts, 28 and 28, will extend the conductors, V and It, oi the line to the wipers 35 and 36; the stoppage of these wipers upon contacts further results in extending the line to the wires, V and R. The movement of the side switch lever,

25, upon its contact, 28 results in placing a ground, through the winding of the relay, DR, upon the wiper, 37, and thus upon the wire, P, thereby preventing an incursion of another first selector upon the trunk which has just been chosen.

Referring to Fig. 4, the wires, V, R and P are shown to enter the circuit of this second selector, at the left of the sheet. Assostrument. Upon his actuation of his circuit controlling device with reference to the second figure of the desired number, the relay, VR, Wlll respond to the succession of ground contacts on the vertical wire, the currentfrom the battery, B, in Fig. 4, passing through the vertical relay, VR, the contact, 4-6", the lever, to the wire, V, thence over the wire, V, to ground at the substation. The relay, VB, will respond in the proper number of impulses, closing its contacts, resulting to operate the magnet, VM, in a corresponding number of impulses, by virtue of current from the battery, through that magnet, the contact of springs, 88 and 39, and springs, 40 and 41, to ground. The vertical magnet will then, through its pawl, lift the shaft, 42, carrying the wipers, 43, 44 and 49, as many steps as there are units in the called digit. As a series of vertical impulses is always followed by one rotary impulse, such a one will energize the relay, RR, by current from the battery, B, through that relay, the contact, 46, the lever, 45,, to the rotary wire, and the substation. The contact made by the relay, ER, in its action serves by current from the battery, B, through the magnet, PM, and the springs, and 41 to ground, to draw up the armature of the magnet, PM. Action on the local circuit of this magnet is immaterial at this time. The important thing is that the armature in drawing up and falling back acts, through its escapement, to advance the side switch levers,

45, to 45 inclusive, one step. Contact upon point, 47, by the lever, 45, establishes a circuit from the battery, B, through the rotary magnet, RM, and its contact, 51, to ground. The contact, 51, is adapted to be broken at the end of each stroke of the armature, so that successive strokes thereof will result, acting through the pawl to rotate the shaft, 42, carrying the springs, 43, 44 and 49, in arcs of the contact bank before which they had stopped at the end of the vertical series. As in a sense the rotation of the shaft is automatic, it will continue until prevented. This prevention results as follows: It will be remembered that when the trunk is busy the private wire P associated with it is grounded. If, therefore, grounds are encountered by the wiper, 49, with the side switch levers in their second posit-ion, current will flow from the battery, B, through the private magnet, PM, the contact, 47*, the lever, 45, through the relay, DR, the wiper, 49, and thence to ground at the contact of P The relay, DR, is of few turns, and of low resistance, and will not be energized by curgrounded contacts of busy trunks.

rent flowing in series with the magnet, PM, which is of relatively high resistance; but the magnet, PM will be energized, and its armature will be held attracted duringsuccessive contacts of the wiper, 49, with the When one is found which is not busy, however, the armature of the relay, PM, through the agency of the escapement, will advance the side switch levers, 45 to 45 inclusive, to their third and extreme position. One result of this motion is to remove the ground from the lever, 45*, stopping further energization of the rotary magnet, RM. Another result is to extend the line wires, V B, to the contacts, 48 and 48, to the wipers, 43 and. 44, and to the trunk conductors, V and B Another result is to place the ground of the contact, 48 through the winding of the relay, DR, upon the contact, 49, and the wire, P thus guarding the selected trunk against incursion on the part of another selector. It will be noted that the vertical and rotary relays, VB and RR, respectively, are no longer connected to the circuit, and that so far as the first and second selectors are concerned, there are no bridges whatever upon it.

Referring now to Fig. 5, the conductors, V B and P enter at the left of the sheet. The relays VH and BB are bridged between the vertical and rotary wires and are both connected with the battery, as shown. Upon the subscriber actuating his impulse transmitting device in response to the third digit of the desired number, the resulting ground contacts upon the vertical wire will operate the relay, VR in as many movements. The result will be the actuation of the vertical magnet, VM by current from the bat tery, B, through the winding of said 1nagnet, through the side switch contact, 56, the side switch. lever, the contact of springs, 59 and 59 and 60 and 61, to ground. This action, through the medium of a pawl will lift the shaft, 62 in as many steps, ending by leaving the wipers, 63, 64 and 65, ready to engage contacts in arcs of the contact bank. As a series of vertical ground contacts at the substation is always followed by one rotary ground contact, such a contact will actuate the relay, BB one impulse. This will cause current from the battery, B, to flow through the private magnet, Pl to the spring, 66, and toground at 61. This will draw up the armature of the magnet, PM and will release it. The action of release, through the agency of the escapemen't, will advance the side switch le vers, 55 to 55 inclusive, one step, leaving them upon their respective contacts, 57 to 57 inclusive. Upon the operation by the subscriber of his mechanism, in response to. the fourth digit of the desired number, a series of vertical impulses of as many units will be given. The relay, VR will respond to current through its winding to those grounds at the substation, and the resulting relay contacts will cause current to flow from the battery, B, through the rotary magnet, Bl the contact, 57 the lever, contacts, 59 and 59 and 60 and 61, to ground. These impulses will, by action of the pawl of the rotary magnet, rotate the shaft, 62, thereby moving the wipers, 63. 64 and 65, in the arcs to which they stood contiguous. The wipers will stop upon the contacts from which lead the wires of the called subscribers line, but connection will not yet be established between the calling and the called subscribers line. The rotary impulse which succeeds the described series of vertical impulses, will actuate the relay, RR causing it to move its springs. Current will flow from the battery, 13, through the private magnet, PM and the springs, 66 and 61, to ground, actuating the private magnet armature in the cycle which advances the side switch by means of the escapement to its third and extreme position. The first result of its reaching this position is to disconnect the rotary magnet circuit, from the lever, 55*. A second result is to extend the line wires, V and R to the wipers, 63 and 64, and thus extend the line from the calling to the called subscriber. It is to be noted that the line now includes the condensers, 66' and 67, one being in series with each wire. A third result of the side switch motion is to place the ground of a contact, 58 upon the lever, 55, and the wiper, 65. This will place ground upon the private wire, P thus guarding the subscribers line from the incursion of another connector.

Referring to Fig. 6, showing the first selector of the line which has been called, the wires, V and R are extensions of the similarly numbered wires in Fig. 5. The wire. P is an extension of the private wire of Fig. 5, and extends through the off normal contacts, 68 and 69, to and through the winding of the relay, CO to the battery. The taking of the line by the wipers, 63, 64 and 65, in Fig. 5, causes current to flow from the battery, B, in Fig. 6, through the winding of the relay, CO and the contacts, 68 and 69, to the ground of the side switch in Fig. 5, as has been indicated. The operation of the relay, CO will break the contact between the springs, 70, 71 and 72, removing the connection of the battery with the relays, VR and BB The bridge formed by these relays is thus taken from the called line and so held during the conversation.

In the sequence of operation at the substation, the ringing button is next pressed to call the desired subscriber. This places ground upon the vertical wire, and by the operation of the relay, VR causes current to flow from the battery, through the magnet of the calling relay, CR, the contact, 58 lever, 55, springs, 59 and 59, and 60 and 61 to ground, energizing the magnet, GR. This will move the springs 7 3 and 74 from their contacts respectively with 75 and 76, and will make contact of 73 and 74 respectively with the terminals of the ringing generator, G. This will cause ringing current to pass upon the called suhscribefis line, and upon his answering conversation may ensue.

In the description of the actions of the connector which has just been concluded, it has been assumed that the line of the called subscriber was not already busy. If it had been busy, the cycle of events which have just been described, would not have been completed in that manner, for the following reason: Upon the completion of the rotary series in which the wipers, 63, 6% and engaged the called line, the succeeding rotary impulse would have actuated the private magnet, PM as has been described; but upon the closinc of the contact between its springs. 77 and 78. current from the battery. B. would have passed through the disconnect magnet. DMZ spring, 78. spring 77. contact, 57. lever. 55, the private wiper, 65, to ground upon the contact of the wire P. This will be so because the side switch is in its second position at this time. The release magnet will thus be operated. and upon its attraction will withdraw the pawls from the ratchets upon the shaft. 62. allowing it to return to normal position by the agency of its spring and gravity. The calling subscriber will not know that this has taken place, and will press his ringing button in the usual manner. This. by placing a ground, upon the vertical wire. will act quite as the usual vertical impul e. causing current to flow through the vertical magnet. the contact. 56 of the side switch which is now in its normal position. the lever. 55*. the springs. 59 and 59 and 60 and 61, to ground. This will lift the shaft. 62, one step, or as many steps as there are pressures upon the ringing button: the 06' normal switch springs. 79 and 80. will close together. Dermittinet current to flow from the secondary winding. 81. of the induction coil. 83. through the lamp. .TJ. the contact. 56. the lever, 55, the callinsr relay contacts, 74 and 76, through the condenser. 67. to the subscribers telephone. and again to the central office. to ground through the relay, V11 and the battery. B: thence through the battery, B, again to the secondary winding, 81. Due to the existence of the primary winding. 82, shunted bv the condenser. 84. and in series with the lamp. L. the interrupter, I. and ground. the battery, B causes undulatory or alternating currents in the secondary winding. 81. This action is assisted by the existence of the condenser, 85, which is in shunt with the interrupter, I. The undulatory or alternating currents thus produced in the circuit described, will pass through the condenser, 67, and will produce in the receiver of the calling subscribers telephone a tone which he will understand to signify that the called line is busy. He will then hang up his telephone, and accomplish disconnection in a manner similar to that which will be described with reference to any usual disconnection at the completion of the conversation.

Dueto the fact that the contact upon springs. 7, at the station of the calling subscriber. has existed throughout the conversation. the hanging up of the telephone will cross together the springs, 12, 13 and 14, placing ground upon both the vertical and rotary wires, at the same time. By reference to Fig. 1, and in view of what has been described, it will be seen that the relays, which can be effected by this simultaneous ground on both wires, are the relays, VH and BB which are permanently bridged across the line at the connector shown in Fig. 5. The relays, BS and ER, are bridged across the line, it is true. but as the line conductors include the condensers, 66 and 67, these relays ire not ini'iuenced by grounds applied at the calling station. The simultaneous operation of the relays, VH and RR acts to close together the springs, 86 and 87. establishing a circuit from the battery, B, through the disconnect magnet, DM the contact between 86 and 87, over the private wire, P to the similarly num- Jered wire in Fig. 4-, to the private wiper, 49, ot the second selector. through the disconnectrela DR. to ground at 48 This current will operate the disconnect magnet, DM releasing the shaft. 62. permitting it to return to normal position, and will as well energize the relay, DR. The connector is now restored to normal position. The relay, DR, institutes by its operation another c cle of events, resulting in current from the battery, 0t Fig. 4, flowing through the disconnect magnet, DM, the springs. 88 and 89, the private wire, P, in Fig. l, the private wire, P in Fig. 3. to the private wiper. 37, the disconnect relav, DR, and to ground at the contact, 28 This accomplishes the release of the second selector shown in Fig. l. through the drawing up and falling away of the armature of the disconnect magnet, DM. The relay, DR, in closing its contact springs, 90 and 91, causes current to flow from the battery, B, through the disconnect magnet, DM, to ground, energizing that magnet, and upon its release of its armature, withdrawing the pawls from the shaft, 32, and permitting it to return to normal position. All of the switches are now disconnected, the side switch being also restored "to its normal position and a new call may be closing together the springs, 92 and 93; and

the cutoff relay is reapplied to the line by closure between springs, 93 and 94.

the called subscriber had desired to disconnect his line from that of the calling subscriber, he might have done so during the connection by moving his calling mechanism as it to institute a call, and then hang;- ing up his receiver. This would have established contact between the springs, '4', and

then. would have closed together the springs, l2, l3 and A, grounding both sides of the line. This would have acted to energize the relays, and ER, simultaneously, quite as it they were vertical and rotary relays, and would have grounded the springs, 95 and 9G, by contact with 9?. Current from the battery, B, in Fig. 5, would then flow through the disconnect magnet, DM to that ground, releasing the connector switch, and leaving the selector of the allcd subscriber, ready to be operated because the breaking of contact between and P in Fig. 5, would have iermitted the cut-off relay CO in Fig. 6, to restore the selector to normal conditions.

In the eyent of the calling subscriber desiring to disconnect the selector switches operated in making a connection before the connector switch is operated, which would be desirable it a mistake were made in op rating the selector switches, they may be returned to normal by depressing the hook switch 3. This simultaneously energizes the relays VH and PR thus closing a circuit from battery B through disconnect battery DM springs 53? and 86, wire P vipers 49, relay DB side switch 45% contact 48 to ground. and thus back to battery B. Since the shaft 652 of the connector switch has not been operated. the energization of the relay Dir P by the above circuit produces no eflcc't. The operation of the relay DR by this cir cuit closes a second circuit from battery 15 through disconnect magnet DM, contacts 88 and 89, wire P, wipers 37, relay DR, side switch 25, contact 28 to round, and thus back to battery 1 The current flowing in this circuit causes the energization of the disconnect magnet DM, thereby releasing the shaft l2 and side switches 45, d5", 45 and 45 of the second selector, thus again connecting the rertical and rotary relays Vlt and HR to the trunk conductors V and R, resnecti'vely. The energiz JlOD of the relay by the circuit last traced closes another circuit from the battery B as follows: battery l3 disconnect magnet DM, contacts 90 and 91 to ground, and thus back to battery B. Th s circuit serves to energize the disconnect magnet DM, and to restore the shaft 32 and side switches 25, 25 25 and 25 of the first selector to normal position, thus again connecting the vertical and rotary relays VI? and to the conductors V and It, respectively. Thus all the apparatus is restored to normal, and the calling subscriber is free to make another call. In effecting the disconnection just described, the private magnet PM is operated by the operation of the relay R11 which results in interrupting the energizing circuit of the vertical magnet VM by the operation of the vertical relay VH From this it appears that the shaft 62 and the associated side switches remain in their normal positions for the disconnecting operation just above described.

For toll line purposes it is practical to establish a toll board in which toll lines center, and which may connect with the automatic mechanism, and the lines therein, by means of trunks which end in first selectors. These trunks would be signaled over by the toll operators quite after the manner of the making of calls by the subscriber. In case such a call were made upon a subscriber, and he desired to re-signal the toll operator for attention at a time when she was not listening upon the circuit, he might do this by pressing his ringing button. This would act to ground the vertical wire as usual; but as the subscriber is one who has been called, this ground will energize the relay, BS, shown in Fig. 5, which will act to make contact between the springs, 97 and 95, grounding the latter with the result of grounding the rotary wire, R The springs 95 and 97 may control a circuit including a signal at the toll board which will actuate to notify the operator whenever the called subscriber presses his ringing button.

In this system the controlling magnets of all the selecting switches except those of the connector, are removed from operative relation with the line during a connection; each of the selector switches actively serving in building: up a connection automatically cuts off the vertical and rotary relay magnets from the line as soon as these magnets have been effective in causing the proper selective movements of the switch. This cutting off of the controlling magnets of the selector switches actively employed in the connection is caused by the movements of the corresponding side switch, which switch also serves to vary the functions of the controlling magnets. The controlling magnets of the first selector of the called line, which plays no part in building up the connection, are removed from the bridge across the line by the action of the bridge cut-0E relay, which is actuated by a connector in making; connection with the corresponding line. The only bridges, therefore, which remain, are those formed by the presence of the vertical and rotary relays, VR and RR of the con nector. and also of the back signal relay, BS, and back release relay, BR, of the connector. It is through the former pair of relays that the calling subscriber is enabled to obtain a complete breaking up of the connection established by him, including the restoration of all of the selector and connector switches involved. Similarly it is through the operation of the latter pair of relays, BS and BR, that a called subscriber is enabled to free himself of a connection which some one else has made with his line, and which he wishes to discontinue.

I do not wish to limit myself in all respects to the exact details and circuit connections shown in this application. For in stance, while I have described a system in this application with particular reference to an exchange wherein subscribers numbers are composed of four digits, it is obviously applicable to apparatus and mechanisms required by systems having a creator or less number of digits in the subscribers desig natino; numbers.

Again, while the system in this application has been described with particular reference to that type of selecting switches in which the selecting contacts have a longitudinal and a rotary motion, it is clear that my invention is equally applicable to other forms of switches in which the selecting of groups and of individual contacts in such groups is caused by other kinds of motions of the selecting contacts.

Having thus described my invention, what I claim as new and desire to secure by Let ters Patent is- 1. In an automatic telephone system, the combination with a telephone line of an automatic switch at the central otlice connected in a conversational circuit with said line, release means for said switch controlled only over said line, a second switch at the central otlice having contacts also in the conversational circuit with said line, and a re lease magnet for said second switch actuated upon the complete release of said first switch, substantially as described.

2. In an automatic telephone system, the combination with a telephone line of step-b step mechanisms at the central office for extendingthe circuit of said line. a second stepby-step mechanism at the central olfice for still further extending the circuit of said line, release magnets for each of said stepby-step mechanisms, the release of one of said step-by-step mechanisms being controlled over said line, and the actuation of the release magnet of the other of said stepbystep mechanisms being effected by the complete release of the first step-by-step mechanism, substantially as described.

Signed by me at Chicago, county of Cook, State of Illinois.

BERT G. DUN HAL/I. 

